In the field of orthopedics, a definite clinical need exists for improved stem extensions for both femoral and tibial components. Each year, more and more aailed primary total knee arthroplasties exist due to loosening, wear, or other reasons. When a total knee arthroplasty fails, removal of the primary components thereof is necessitated, which commonly results in considerable bone loss necessitating a stem fixation in the subsequent implant.
Clinical experience has shown that large stem extensions act to stabilize the prosthesis through transmission of surface stresses to the stem and consequently to the cortical bone, thereby stress shielding deficient bone in the immediate surface bearing area. Consequently, as surgical procedures evolve, larger and longer stems are being proposed for revision surgery.
The problem is complicated because anatomical variations in different patients require a surgeon to carry in inventory four to six sizes of femoral components and ten to fifteen sizes of tibial components. Further complexity is added by virtue of the fact that the femoral stem has to be at a 6 degree valgus angle to the femoral component to fit the intermedullary canal of the femur. Thus, these prosthetic components require a great number of fixed stems. One manufacturer, for example, has at least nine different femoral stem sizes ranging from 10 millimeters to 22 millimeters in diameter. Considering the fact that they also make six femoral prostheses, for each of the left and right-hand sides, it is easy to see that over one hundred sizes and shapes of fixed prostheses have to be manufactured and must be carried in inventory by the surgeon or hospital. The high expense of each prosthesis makes carrying such a large inventory of prostheses exhorbitantly expensive.
If a stem extension could be created which could fit on the undercarriage of the articular surface of any one of a multiplicity of prostheses and, further, if such a stem extension could be made reversible, the above described inventory of prostheses could be reduced to about six prostheses and nine reversible stem extensions. Clearly, having to carry an inventory of fifteen pieces is far superior to the requirement now present in the art of keeping an inventory of over one hundred prosthetic combinations.
In a further aspect, there are anatomical considerations in the design of a stem extension which make it desirable to have such a stem extension angled in the lateral plane as well as in the medial lateral plane. Ideally the distal femoral cut of the femur is at 90 degrees or perpendicular to the angular bow of the femur as seen in the lateral plane. The angle of bowing of the femur may vary from 0 up to 12 degrees. Most commonly, this angle is from 3 to 6 degrees. Thus, fitting a fixed straight stem extension into the intermedullary canal when it has a significant degree of bowing may be quite difficult. However, if a stem extension could be made which was angled with respect to the axis of the prosthesis, such a stem extension could be installed in the intermedullary canal more easily.
Further, additional complications exist in the inclusion of a stem extension in a tibial component. As is known, some instrumentation systems which are designed to measure the exact location of tibial cuts result in cutting of the top of the tibia at varying angles such as, for example, 10 degrees 7 degrees, 3 degrees, and some absolutely perpendicular. Thus, a properly designed stem extension could be used to modify a tibial component to the particular method of instrumentation at minimal cost. While most surgeons use a 3 degree sloping cut on the proximal tibia, applicant's surgical group has been using a 10 degree slope for a number of years and finds it to be advantageous in obtaining more flexion and in controlling the forward movement of the femur on the tibia when the patient is descending stairs and making other similar movements. In such a situation, of course, the prosthesis could be initially installed without the stem extension and, if a refit is necessitated, the stem extension could be easily installed.
The following prior art is known to applicant:
U.S. Pat. Nos. 4,404,691 to Buning, et al. and 4,578,081 to Harder, et al. disclose a modular prosthesis assembly "including a mounting component provided with a connection portion and at least two joint components of similar shape but different dimensions and which can be connected alternatively to the mounting component". The present invention differs from the teachings of these patents as providing a different means of attachment and different characteristics as to angular displacements in the various planes as compared to the teachings of these patents.
U.S.S.R. Pat. 797,680 discloses a prosthesis including a stem and a head and wherein interconnection therebetween is by virtue of a pin. This is different from the teachings of the present invention, wherein a modular system is provided with the attachment being by virtue of two pins which are of different configuration and orientation from that which is disclosed in this U.S.S.R. Patent.
European Patent 0 021 407 to Montagne discloses a femoral prosthesis wherein a single stem assembly may be attached to one of a plurality of heads by virtue of one of a multiplicity of stem extensions. This is different from the teachings of the present invention, wherein a modular approach is utilized which reduces the number of stem extensions which may be required and which has a means of attachment different from that which is disclosed in this European Patent.